Halogen metal vapor high-pressure discharge lamps have a fill which can ionize. For generation of visible light, the fill uses halides of sodium and/or tin. To use high-pressure halogen metal discharge lamps for medical or technical applications, and all others in which, essentially, radiation should be in the ultraviolet spectral range, the ionizable fill includes one or more halides of mercury, iron and/or nickel.
Lamps having halogens in their fill are subject to two halogen cycles. One of them prevents blackening of the discharge vessel; another halogen cycle, however, occurs which affects the electrode material. This second cycle damages the electrodes and leads to substantial problems of quality in the lamps, due to corrosion of the electrodes or the electrode stems.
The electrode material usually is tungsten or thoriated tungsten. The vaporized tungsten halide or, for example, a tungsten-oxygen halogen combination, dissociates during the discharge. The tungsten which is thus liberated is derived, at least in part, from the electrode and the electrode shaft and precipitates at the hot spots of the electrodes, or on the tip of the electrodes. The electrodes could break by corrosion at a portion of the electrode or electrode shaft which is weakened. Breakage of the electrode, of course, leads to failure of the lamp. FIG. 2 illustrates, schematically, the reaction diagram which leads to electrode corrosion.
Free oxygen (O.sub.2), together with tungsten, forms tungsten dioxide (WO.sub.2) which reacts with a halogen (X.sub.2) in the discharge vessel to form a tungsten oxide halide (WO.sub.2 X.sub.2). The tungsten oxygen halogen compound dissociates in the discharge, schematically shown at D. The resulting tungsten deposits at the hot spots of the electrodes. The oxygen (O.sub.2) and the halogen (X.sub.2) are available at the cooler portions of the electrodes, or portions or parts thereof, from which tungsten degradation or removal occurred, providing further electrode material (W) for continuation of the cycle process.
Halogen metal vapor high-pressure discharge lamps which have metal halide fills primarily containing sodium halide or tin halide are particularly affected by this electrode corrosion. Likewise, ultraviolet (UV) radiation sources having metal halide fills which primarily include mercury halides, iron halides and/or nickel halides, likewise are substantially affected.
The problem of electrode corrosion, heretofore, has been solved by adding excess metal to the ionizable fill of the halogen metal vapor lamp. The excess metal binds free halogens, so that the participation of electrode material in the halogen cycle is substantially limited. For example, and using an atomic metal/halogen ratio which is greater than or equal to 1:5, lamp lives of more than 6000 operating hours have been reached. This is described in the referenced publication "Technisch-wissenschaftliche Abhandlungen der OSRAM-Gesellschaft" ("Technological-scientific papers of the OSRAM company), Vol. 12, published by Springer, Berlin, Heidelberg, New York, Tokyo, 1986, pp. 65-72, article by D. C. Fromm: "Elektrodenentwicklung fur kleine Halogen-Metalldampflampen" ("Electrode Development for Small Halogen Metal Vapor Lamps").
U.S. Pat. No. 4,633,136, Fromm, assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference, describes, generally, a halogen metal vapor discharge lamp of the type to which the present invention relates. The ionizable fill in this lamp has an excess of tin in order to prevent electrode corrosion. Further, a surrounding winding protects the electrodes at the cooler regions within the melt connection zone, since these regions are particularly affected by electrode corrosion. The windings surround at least one of the electrode shafts.